class TangoExpr(list):

    @property
    def is_unit(self):
        return len(self) == 0

    @property
    def is_block(self):
        return len(self) > 0 and isinstance(self[0], TangoExpr)

    @property
    def is_call(self):
        return len(self) > 0 and isinstance(self[0], TangoIdent)


class TangoIdent(str):

    def __repr__(self):
        return super().__str__()